Second SELECT query if first SELECT returns 0 rows

Question

I am trying to speed up a PHP script and I am currently pushing some PHP logic in the Mysql domain of the thing. Is there a way to make a different select query if the first Select returns no rows, or a count of zero ?

Keeping in mind that the first query needs to run first, and the second should only be activated if the first one returns an empty set.

SELECT * FROM proxies WHERE (A='B') || SELECT * FROM proxies WHERE (A='C')

For the above 2 queries I have this code, but it seems to run each query twice (once to count, and once to return). Is there a better way to do this?

IF (SELECT count(*) FROM proxies WHERE A='B')>0
    THEN SELECT * FROM proxies WHERE A='B'
ELSEIF (SELECT count(*) FROM proxies WHERE A='C')>0
    THEN SELECT * FROM proxies WHERE A='C'
END IF

Answer 1

One option would be to use UNION ALL with EXISTS:

SELECT * 
FROM proxies 
WHERE A='B'
UNION ALL
SELECT * 
FROM proxies 
WHERE A='C' AND NOT EXISTS (
    SELECT 1
    FROM proxies 
    WHERE A='B'
)

• SQL Fiddle Demo

This will return rows from the proxies table where A='B' if they exist. However, if they don't exist, it will look for those rows with A='C'.

Answer 2

SELECT * 
FROM proxies 
WHERE A=(CASE WHEN
             (SELECT COUNT(*) FROM proxies WHERE A='B') > 0 THEN'B' 
              ELSE 'C' END)

UPDATE

SELECT * 
FROM proxies 
WHERE (
    CASE WHEN (SELECT COUNT(*) FROM proxies WHERE A='B' LIMIT 1) > 0 THEN
            (A='B')
         WHEN (SELECT COUNT(*) FROM proxies WHERE A='C' LIMIT 1) > 0 THEN
            (A='C')
         WHEN (SELECT COUNT(*) FROM proxies WHERE A='D' LIMIT 1) > 0 THEN
            (A='D')
         ELSE 1=2 END)